Both institutional and consumer automatic dishwashers or warewashing machines have been in use for many years. These dishwashers typically function with two or more cycles, including various combinations of a soak or prewash, a main wash, a rinse, a sanitize and a dry cycle. A dishwasher detergent composition is typically utilized during the wash cycle to remove soil and stains. Often, the detergent composition will include water softeners, bleaching and sanitizing agents, and an alkali source.
For many reasons, separate rinse additives or aids are an important part of the automatic dishwasher operation. In general, rinse aids minimize spotting and promote faster drying, by causing the rinse water to sheet off of the clean dishes evenly and quickly. Rinse aids are generally used in a cycle separate from cycles using the detergent composition, although some detergent residue may be present in the rinse water.
Rinse aids are currently available in liquid or solid form. The use of a solid rinse aid can be much preferred. Solid rinse aids can be more convenient, safe and economical than liquids because they do not spill or splash. In addition, dispensers for solid rinse aids tend to be less expensive and more durable because generally they have no moving parts. However, many surfactants with good rinse performance are commonly available only in a liquid or paste form at room temperature. The invention provides solid rinse aids from liquid, paste-like, or solid surfactants.
Solid rinse aids are available for consumer and institutional warewashing machines. For use in a typical consumer machine, each solid rinse aid generally incorporates a disposable container or basket which is hung directly inside the machine. This container is also referred to as a dispenser. Circulation of water within the machine in the normal course of the machine cycles slowly dissolves the solid rinse aid, thus dispensing it. The water temperature in consumer machines typically falls between 60.degree.-180.degree. F.
Institutional machines are generally either low temperature machines with a water temperature of from about 120.degree.-140.degree. F., or high temperature machines with a water temperature of about 160.degree.-180.degree. F. A low temperature warewashing system can be more desirable than a high temperature system because it avoids the heating expenses associated with the hotter water. In addition, it is much simpler to dispense a rinse aid in a low temperature system. In a low temperature system, a quantity of rinse water can be added to the sump of the automatic dishwashing machine and circulated to rinse the dishes, before draining. In such a system, the rinse aid need only be provided to the sump, and will function as the water circulates.
By contrast, in a higher temperature system dissolved rinse aid is injected into the rinse water line prior to entering the machine and is then sprayed over the dishes from a rotation spray arm. A continuous stream of hot water is commonly provided through the spray arm for rinsing. Consequently, a rinse aid for use in a high temperature system must be dispensed into and sufficiently dissolved in the hot water stream against a back pressure before the water leaves the spray arm and contacts the dishes. This generally requires a more complex dispensing system.
There are two aspects to surfactant solubility which must be considered in the context of a solid rinse aid. First, the surfactant itself must be sufficiently water soluble to function as a rinse aid. This requires a surfactant solubility of at least about 5-10 ppm, or more commonly, about 40-80 ppm in water somewhere between 60.degree.-180.degree. F. depending upon the warewashing system. Many surfactants meet this requirement.
However, some slid surfactants, which in view of their solubility and performance could be very effective rinse aids, are not in use because their low water solubility prevents effective dispensing. This illustrates the second and more important aspect of solubility, namely, the surfactant must be soluble enough to dispense in an effective quantity during the short time that water impinges the solid to dispense it. For example, a solid surfactant may be soluble enough to function as an effective rinse aid if an appropriate amount were dissolved in the rinse water; however, if an attempt were made to dispense the solid into the rinse water in the typical way, that is, by solubilziing a portion through impingement with a brief water spray, the solid may not solubilize quickly enough to be useful. In the context of this invention, the solid rinse aid (which may have been formed of a solid, paste-like, or liquid surfactant according to the invention) is soluble enough to dispense in an effective amount, even if the surfactant alone would be too insoluble for effective dispensing.